Streamline implementation of GetUniqueIntID:

- if available, always use the GCC intrinsic function
- otherwise fall back on OS provided atomic operations
- TODO: support other platforms (e.g. Intel compiler)
Updated benchmark to also run the GetUniqueIntID function
Adding some unit tests, these are just the demos right now but it's a starting point at least
disable mitsuba test for now
Disable warning C4275 in Windows MSVC compiler, that generated tons of warning since we splitted the unit_IRRLICHT into .h and .cpp. Makes the compilation log of this unit less verbose.